Eurozone finance ministers failed to agree on a deal which would have released vital rescue funds for Athens on Monday night, after Greece's creditors rejected calls for an upfront commitment to reduce the country's debt burden.

Jeroen Dijsselbloem, who leads the Eurogroup of finance ministers, said the ministers had held an "in-depth discussion" on debt sustainability and said they were "very close" to an agreement.

However, he added that they had "not reached an overall agreement on that part of the discussion".

"Tonight we were unable to close a possible gap between what could be done and what some of us had expected should be done or could be done. We need to close that by looking at additional options or by adjusting our expectations.

"Both are possible and both perhaps should be done, and that I think will bring us to a more positive and definite positive conclusion at the next Eurogroup in June," Mr Dijsselbloem said.

Talks are expected to continue over the coming weeks ahead of the next meeting on June 15.

Prior to the meeting, Eurozone finance ministers had said they were confident that a political agreement could be reached on Monday evening. This would have paved the way for a fresh tranche of financial aid to ensure Greece avoids a summer cash crunch.

However, officials were at odds with the International Monetary Fund (IMF) over the critical issue of debt relief, which is a condition of the Fund's participation in Greece's third, €86bn (£74bn) bail-out.

The IMF had stressed that debt relief was necessary to ensure the country can return to fiscal health, and had called for details on the scope and timing of relief before it joined the programme.

Ahead of the meeting in Brussels, Mr Dijsselbloem had said he was optimistic that creditors would release new loans to Athens after the Greek parliament passed fresh austerity measures last week, including pension cuts.

Greece's debt share currently stands at around 180pc of gross domestic product (GDP), but Mr Dijsselbloem said detailed relief measures would not be thrashed out until 2018.

Insiders said talks aimed at bridging the gap between the IMF and some of Greece's creditors would be difficult.

"Discussions are going to be long, and I am not sure they will be successful," said one.

Others said everyone was working hard to secure a deal that included the Fund. "If we lose the IMF now, we lose the IMF forever," said one source.

Wolfgang Schaeuble, Germany's finance minister, had also been optimistic that ministers would "conclude things politically", enabling Greece to repay a series of creditors this summer, including the European Central Bank.

Pierre Moscovici, European Commissioner for economic and financial affairs, said Brussels' report on Greece's compliance with the terms of the rescue package was also "positive", supporting the case for the €6bn disbursement.

However, Mr Schaeuble insisted that Germany - which faces elections this autumn - could not meet the IMF's demands until the end of the programme in 2018.

Bruno Le Maire, France's newly appointed finance minister, had been more open to granting Greece debt relief.

Speaking ahead of the meeting, he said he "saluted" the difficult decisions made by the Greek parliament on further austerity measures.

"These are significant efforts demanded from them and I think that it makes it even more necessary that we find an agreement today in the Eurogroup... an agreement to allow Greece to envisage its future in a more positive manner."

It came as Portugal, which received a €78bn international bail-out in 2011, was declared back to fiscal health by the European Commission in a "turning point" for the eurozone economy.

The country's budget deficit fell to 2pc of GDP last year, taking it below the EU's limit of 3pc and releasing it from Brussels' excessive deficit procedure, which left the country open to censure and fines.